Patients whose illness requires chronic transfusions can develop so many different alloantibodies to RBC antigens that no crossmatch-compatible RBC units can be identified despite nationwide searches. When this occurs, patients are forced to forgo the therapeutic, and, in some cases, lifesaving benefits of RBC transfusion. Thus, chronically transfused patients, such as those with sickle cell disease and other chronic anemias, suffer morbidity and mortality from alloimmunization against multiple blood group antigens. Currently, no therapeutic interventions can circumvent the situation of multiple alloantibodies and allow for transfusion of crossmatch-incompatible blood. However, anti-RBC antibodies can be modified to maintain their antigen binding activity but lose activities required for hemolysis, such as the ability to fix complement or bind to Fc Receptors on phagocytes. We hypothesize that these modified antibodies will mask the offending epitopes, thereby blocking RBC hemolysis by intact IgG. Thus, we propose to develop a novel therapeutic approach by engineering such antibodies, which we refer to as epitope masking reagents (EMRs). Our central hypothesis is that the use of EMRs will allow transfusion of crossmatch- incompatible RBC by masking epitopes recognized by antibodies responsible for hemolysis. To allow for a detailed mechanistic development of this approach, we created a novel murine model for studying crossmatch-incompatible transfusions. Using this model, we can 1) obtain RBC with a single antigenic difference between donor and recipient mice, 2) immunize mice to the blood group antigen on the donor RBC, 3) transfuse packed, leukoreduced, donor RBC by an intravenous route, 4) detect in vivo antibody binding to transfused RBC, and 5) monitor hemolysis and circulatory lifespan of transfused RBC. We propose to use this model of crossmatch-incompatible transfusion as a platform to develop and test the efficacy of EMRs. Patients requiring many blood transfusions are exposed to many blood donors. Therefore, it may eventually become difficult to find compatible blood for them. In this case, they are at risk for a transfusion reaction if they are transfused with incompatible blood, but are also at risk if they are not transfused. We propose to develop novel therapies that would allow transfusions of incompatible blood to patients in this setting. [unreadable] [unreadable] [unreadable]